1. Field of the Invention
This invention relates to an imaging device and, more particularly, to a solid-state imaging device having a semiconductor substrate on which a plurality of light receiving elements and the like are integrated.
2. Description of the Related Art
The solid-state imaging device is required to have a resolution comparable to that of an electron tube for imaging used in presently commercialized television broadcasting. To this end, it is necessary to form on the semiconductor substrate a photocell matrix in which 500 photocells (photoelectric conversion elements) are arrayed in the vertical (column) direction and 800 to 1000 photocells are arrayed in the horizontal (row) direction and scanning elements for scanning these photocells. Accordingly, such a solid-state imaging device is fabricated using MOS.LSI technology capable of realizing high density of integration and charge coupled devices (CCD's) or MOS transistors are generally used as constituents of the solid-state imaging device.
As an example, a MOS type solid-state imaging device employing switching transistors of SOI (silicon on insulator) structure will be described hereinafter.
This type of solid-state imaging device is disclosed in IEDM 86, Technical Digest, pp. 369-372. This prior art example will be explained with reference to FIGS. 1 and 2 in the accompanying drawings. FIG. 1 is a diagram illustrating the circuit construction of the MOS type solid-state imaging device. As shown, a plurality of photodiodes 101, 201, 301 and 401 are arranged in matrix form within a p-type silicon substrate and MOS transistor switches 104, 204, 304 and 404 for selecting individual photodiodes are respectively connected to corresponding photodiodes. A set of one photodiode and one MOS transistor switch constitute one photocell. Signal lines 510 and 511 for delivery of signal electric charges are respectively connected to output terminals of the vertical (column) direction MOS transistor switches 104 and 304 and to output terminals of the vertical direction MOS transistor switches 204 and 404. The MOS transistor switches 104, 204, 304 and 404 are sequentially scanned with signals on gate lines 512 and 513 connecting to a shift register 514. This sequential scanning ensures that a signal electric charge generated by light incident upon the photodiode 101, 201, 301 or 401 is delivered through the MOS transistor switch 104, 204, 304 or 404 and the signal line 510 or 511.
FIG. 2 is a diagram illustrating the sectional structure of one photodiode and its neighborhood shown in FIG. 1. Formed in the p-type silicon substrate designated by reference numeral 17 is an n-type inpurity-doped region (n.sup.+ layer) 15 forming part of the photodiode and isolated by oxide films 12, and p-type impurity-doped regions (p.sup.+ diffusion layer) 11 also formed in the p-type silicon substrate 17. The oxide film 12 and p-type impurity-doped region 11 form an isolation region which isolates photocells (pixels) from each other. One MOS transistor having a source 16, a drain 6 and a gate 7 extends from the n-type impurity-doped region (n.sup.+ layer) 15 so as to overlie the oxide film 12, and a signal line 8 is connected to the drain 6. The resulting structure is covered with a protection film 14. Particularly, the MOS transistor having the source 16, drain 6 and gate 7 is formed within a SOI structure which is of recrystallized silicon prepared using the n-type impurity-doped region (n.sup.+ layer) 15 as a seeding region. Accordingly, the parasitic capacitance between drain 6 and p-type silicon substrate 17 can be minimized and consequently the parasitic capacitance associated with the signal line 8 can be minimized to suppress the output noise level. This is due to the fact that square of output noise current is proportional to the total value of capacitance associated with the signal line 8. The MOS type solid-state imaging device of the above construction is also disclosed in U.S. Pat. No. 4,551,742.
Through experiments, the present inventors have found that leakage current flowing between the source 16 and drain 6 of the MOS transistor formed within the SOI used in the prior art device is far larger than leakage current flowing through the source and drain of a MOS transistor formed by way of trial in the silicon substrate 17, and that there arises an unsolved problem that the leakage current in the MOS transistor of SOI structure leads to generation of additional noise components and loss of the signal electric charge. This problem bottlenecks putting the MOS type solid-state imaging device of SOI structure into practice.